As an example of the sheet-like tire constituting member, there is a tread rubber. The sheet-like tire constituting member is tabular in an expanded state as shown in FIG. 6(a), however, when being formed in a tire shape, it is necessary to join an end portion and an end portion on a forming drum. However, if an overlap is generated or a gap is generated when the end portions are joined with each other, uniformity is lowered, and there is a risk that a tire performance is adversely affected (refer to FIGS. 6(b) and 6(c)).
Accordingly, it is necessary to form front and rear end portions of the sheet-like tire constituting member so that a thickness thereof is changed little by little, as shown in FIG. 7(a). As a result, the end portions can be overlapped with each other so that a thickness is entirely uniform, as shown in FIG. 7(b). In order to achieve the formation of the rubber having the shape as mentioned above, there has been a known rubber layer forming method by a variable die head which is disclosed in the following patent document 1.
Specifically, it is disclosed in FIG. 6 of the patent document 1. The variable die head is controlled so as to change a dimension in a width direction of an opening of the die head little by little. Further, in order to form a tread portion which has a comparatively large thickness, the tread portion is formed by overlapping several layers of rubbers each having a small thickness.